U.S. Pat. No. 3,879,660 which issued Apr. 22, 1975 to John S. Piso discloses a system for continuously measuring changes in materials and, specifically, for monitoring denier of synthetic yarns. The yarn passes through the plates of a pair of capacitors connected in a capacitance bridge. The bridge unbalance output is differentially amplified and fed to a synchronously operated phase sensitive detector. The direct current output of the detector is utilized as an indicator of denier.
The apparatus disclosed in the referenced patent has proved to be a commercially successful device. However, synthetic yarns tend to pick up moisture from the atmosphere and the presence of such moisture in the capacitor dielectric may alter the bridge balance, thereby resulting in a false denier measurement. The prior art has disclosed or suggested the use of capacitance transducers at plural frequencies for the measurement of moisture and other variables. However, these prior art disclosures involve a relatively complex mathematical treatment of the output signals and, in most instances, utilize the output ratio to derive desired information. Examples of such disclosures will be found in U.S. Pat. Nos. 3,155,898 of H. R. Chope; 3,155,900 and 3,155,901 of A. F. G. Hanken; 3,155,902 of G. W. Walls; 3,241,062 of C. W. Baird; and 3,290,588 of A. Norwich. Accordingly, it is a primary object of the present invention to provide simpler method and apparatus for employing plural frequency capacitance transducer measurements of yarn denier corrected for moisture. Other objects are: to provide such method and apparatus which does not require ratioing or complex mathematical treatment of signal outputs; which may be precalibrated for each type of yarn; and which yields accurate measurements of yarn denier and denier variability regardless of the moisture contained in the yarn. The manner in which these objects are achieved will be more apparent from the following description and appended claims.